1. Field of the Invention
The present invention relates to a system and a method for improving the print quality of a printer. Specifically, the present invention discloses a system and method for adjusting the timing interval between a print head stepping signal and a print head firing signal so that pixels are formed at desired locations.
2. Description of the Prior Art
The increasing sophistication of computer systems has lead to a corresponding increase in the graphical resolutions of these systems. Computer monitors are displaying more pixels with more color, and scanners are scanning documents at more pixels per inch than ever before. There is, therefore, an equal demand placed upon printers to offer extremely high-resolution printing. A direct consequence of this is that finer tolerances are placed upon the print head driving systems of these printers.
Please refer to FIG. 1. FIG. 1 is a perspective view of a prior art printer 10. The prior art printer 10 has a carrier 9 that is slidably disposed on a print track 7. The carrier 9 can move forward and backward, which is indicated by the arrow FB. The carrier 9 is used to hold a print cartridge 6, which is removably fixed in the carrier 9.
Please refer to FIG. 2, in conjunction with FIG. 1. FIG. 2 is a block diagram of the prior art printer 10. The cartridge 6 has a print head 20. The print head 20 does the actual printing, jetting ink onto a document. The print head 20 comprises a plurality of orifices 22 that are used to jet ink onto the document. Generally speaking, the orifices 22 are arranged in rows and/or columns and can jet ink of different colors. For the sake of simplicity, the following discussion will concentrate on only one of the orifices 22. It should be born in mind, however, that the methods and systems discussed are all equally valid and designed for the full plurality of orifices 22.
The prior art printer 10 further comprises a control circuit 30 and a driving system 40. The driving system 40 comprises a stepping motor 42 that is controlled by a stepping integrated circuit (IC) 44. The stepping IC 44 provides electrical signals 46 to control the stepping motor 42. The driving system 40 is mechanically connected to the print head 20 to move the print head 20 along the print track 7. This mechanical connection is indicated by arrow 40d. The control circuit 30 controls the general operations of the printer 10. In particular, it sends a control signal 30c to the driving system 40 to trigger a stepping function of the stepping motor 42, and sends a firing signal 30f to the print head 20 to make the orifice 22 jet ink. In this manner, the control circuit 30 can get the print head 20 to move to a particular location and form a pixel at a desired pixel location.
Please refer to FIG. 3 in conjunction with FIGS. 1 and 2. FIG. 3 is a simple schematic diagram of the stepping motor 42. Please note that the structure of the stepping motor 42 has been greatly simplified. The stepping motor 42 comprises a rotor 43, a stator 45, and two pairs of coils wound on the stator 45. By supplying current to alternating coils on the stator 45, the rotor 43 can be made to rotate through succeeding 90 degree steps. With the configuration shown in FIG. 3, each 90 degree rotation of the rotor 43 is called a full-step. Thus, to create a full-step, current is turned off for the present pair of coils on the stator 45 and is turned on for the succeeding coils on the stator 45. Under this shifted magnetic field, the rotor 43 will rotate to align with the corresponding energized coils on the stator 45. As noted above, it is the stepping IC 44 that generates signals 46 to control the stator current.
It should be clear that not only full-steps are possible for the stepping motor 42. It is also possible to perform a half-step. To perform a half-step, the stepping IC 44 generates signals to supply current equally to both pairs of adjacent stators 45. From a vertical or a horizontal position, the rotor 43 will rotate 45 degrees, balancing between the equal magnetic fields generated by the adjacent stators 45. Current is then turned off for the preceding pair of stators 45, and the rotor 43 will make another 45 degree rotation, completing a full-step. In this manner, accurate half-stepping of the rotor can be achieved. Furthermore, steps finer than half steps can be achieved by varying the ratio of the stator current between adjacent pairs of stators 45. Such steps, finer than a half step, are termed micro-steps. It is the job of the stepping IC 44 to provide these carefully calibrated stator currents to provide accurate micro-stepping of the rotor 43. The stepping IC 44 may generate signals 46 to advance the stepping motor 42 by one micro-step when receiving proper control signals 30c from the control circuit 30.
By providing micro-stepping, the overall resolution of the stepping motor 42 is greatly increased, which directly leads to a finer pitch when printing. This is illustrated in FIG. 4 and FIG. 5. FIG. 4 is a phase diagram of angular displacements for micro-stepping of the stepping motor 42. FIG. 5 illustrates locations of the print head 20 resulting from each micro-step of FIG. 4. In FIG. 4, the micro-step number is indicated by an encircled numeral. For the stepping motor as shown in FIG. 3, each full step has been broken into 16 micro-steps, with the intermediate steps running from 1 to 15. Ideally, the angular rotation of the rotor 43 from one micro-step to the next should be 90xc2x0/16, which equals 5.6250xc2x0. Depending on the gearing of the driving system 40d, each of these micro-steps should be translated into an equal displacement of the print head 20 along the print track 7, such as {fraction (1/1200)} of an inch for a 1200 dpi printer. These displacements are indicated in FIG. 5, with the resulting location of each micro-step on the print track 7 indicated by its encircled numeral.
In the prior art, the control circuit 30 comprises a timer 32. The timer 32 is used to generate regularly spaced control signals 30c that are sent to the driving system 40. The interval between control signals 30c is of a sufficient length of time to enable the rotor 43 to move to and settle into the next micro-step position. The control circuit 30 then sends out the firing signal 30f, and the firing signal 30f will logically xe2x80x9cANDxe2x80x9d with the image data to activate the orifice on the print head to jet the ink. In other words, the print head will jet the ink if both the firing signal 30f and the image data are xe2x80x9c1xe2x80x9d, and will not jet the ink if either one of the firing signal 30f or the image data is xe2x80x9c0xe2x80x9d. Thus, the same interval xcex94t exists between successive firing signals 30f and successive control signals 30c, the two signals having only a constant time delay between them. The timing of the control and firing signals is indicated in FIG. 6. The result of these two signals 30c and 30f, in conjunction with the even micro-steps of the stepping motor 42, should result in pixels placed at evenly spaced intervals. That is, with each successive micro-step, a pixel should be formed on a desired pixel position 23 that corresponds to that micro-step, as indicated in FIG. 5.
The above is the ideal. The reality is that the stepping IC 44 is unable to evenly divide the angular distribution of the micro-steps between full-steps. This problem is illustrated in FIG. 7. FIG. 7 is a phase diagram of the actual angular displacements for the micro-stepping of the stepping motor 42. The stepping IC 44 uses some approximation technique (e.g. linear approximation) to map the arc of the full-step. This results in some of the micro-steps making too large of a rotation, and others making rotations that are too small. This irregularity in the angular distributions of the micro-steps results in a corresponding irregular distribution of the position of the print head 20 at each micro-step. Consequently, the actual printed pixel locations do not land on the desired pixel locations. This is illustrated in FIG. 8, which contrasts desired pixel locations with actual pixel locations, with 0.01 inches per full-step and 16 micro-steps per full-step.
It is therefore a primary objective of this invention to provide a method and system for forming pixels on desired pixel locations by adjusting the relative timing between the control signal and the firing signal.
The present invention, briefly summarized, discloses a method and corresponding system for improving the print quality of a printer. The printer has a print head for forming a pixel, and a driving system for moving the print head from a first location to a second location. The print head forms the pixel according to a firing signal. The movement of the print head is controlled by a control signal sent to the driving system. The method involves building a list of desired pixel locations, building a calibrated list of firing signal offsets, sending the control signal to trigger movement of the print head, and sending a firing signal to the print head to form a pixel at a predetermined location. The firing signal offsets correspond to the desired pixel locations, and are adjusted to compensate for the driving system. The timing of the firing signal is determined by the timing of the control signal and by a firing signal offset in the calibrated list of firing signal offsets. The firing signal offset adjusts the firing time so that the predetermined location of the pixel is effectively on a corresponding desired pixel location.
It is an advantage of the present invention that by carefully adjusting the time interval between the sending of the control signal and the sending of the firing signal, variations in the driving system of the print head are compensated. Specifically, variations in the angular movement of the micro-stepping of a stepping motor can be considered. Pixels are therefore formed on their respective desired locations.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings.